The invention relates to a vessel provided with a retractable thruster that is mounted in a frame which is displaceable in a shaft in the hull of the vessel, which frame is connected to a lifting device for retracting the thruster in two stages, the lifting device in the first stage lifting the thruster to a level at or near the keel of the vessel, the second stage comprising lifting the thruster to an access level, located above the first level.
Vessels with retractable thrusters are known, wherein one or more thrusters can be extended up to about 4 metres below keel level of the vessel. These thrusters are often used for dynamic positioning purposes and for maintaining the vessel in a fixed orientation with respect to prevailing wave and current directions. However, while sailing to a working location or while entering a harbour, the positioning thrusters are retracted flush with keel level of the vessel in order to decrease the water resistance and to not interfere with the sea bed in shallow waters.
The known retractable thrusters are relatively difficult to inspect and to maintain as maintenance can only be carried out in a dry dock or by means of divers. Furthermore, the known thrusters, which may comprise an integrated containerised unit having a drive control unit, a propeller, cooling etc., take a long time to install and can only be mounted with relative difficulty.
From FR-A-2 560 147 a vessel having a retractable thruster is known. In the known construction, the thruster drive motor is mounted in a tank which comprises a rack which can be lifted by means of pinions on the vessel. The pinions are mounted in a movable frame which can be locked in different positions on a lifting structure mounted on deck level for staged retraction of the thruster.
In an alternative embodiment, the rack and pinions are replaced by a hydraulic cylinder (FIG. 11) mounted on the movable frame.
The known thruster can be retracted in two stages, the lifting device in the first stage lifting the thruster to a level at or near the keel of the vessel, the second stage comprising lifting the thruster to an access level, located above the first level. By mounting the thruster for instance through a hole in the vessel (moonpool-like), the thruster can be retracted in the first stage to be flush with the hull of the vessel for purposes of entering shallow waters or for sailing the vessel at high speed. In case maintenance is necessary, the thruster is lifted to the second level for easy access via for instance a tween deck. By mounting the thruster, that is preferably connected to a frame or container, in a hole (moonpool-like) in the vessel, a relatively simple construction is obtained wherein the thruster can be put in position when the vessel is already afloat. Hereby relatively short construction times can be achieved.
The known lifting device has as a disadvantage that a relatively large lifting structure is permanently mounted on deck level, which may during offshore operations form an obstacle for equipment placed on deck level. In the first stage of retraction of the thruster, a large part of the lifting device that is connected to the frame extends above deck level.
It is therefore a purpose of the present invention to provide for a relatively compact retractable thruster that can be easily installed and on which maintenance can be carried out in an easy manner, the lifting device in the first stage of retraction occupying relatively little space on the deck of the vessel. It is a further object of the present invention to provide a compact system that is of simple design and that is particularly suited for conversion of existing vessels to vessels comprising one or more thrusters for dynamic positioning purposes.
Thereto, according to one embodiment of the present invention, the shaft of the vessel is provided with a rack. The frame comprises a pinion movable along the rack, wherein in the second stage, the rack can be extended by means of a separate rack extension means which is attachable to the rack. By means of for instance a double-toothed rack which is connected to the vessel and by pinions connected to the thruster frame, the thruster can be retracted to keel level without any significant structure projecting along deck level. For maintenance purposes, the rack extension means is temporarily connected to the main deck so that the track for the pinions is extended and the thruster can be displaced along the second stage up to the level of a tween deck. A temporary workfloor can be constructed across the shaft in the vessel wherein the thruster can be displaced for access of a maintenance crew to the thruster.
In an alternative embodiment, the lifting device may be comprised of hydraulic cylinders that are placed below deck level at a fixed position. The frame of the thruster is detachably coupled to the lifting device, or cylinders. The thruster can be lifted to the first stage at for instance a full stroke of the cylinders, and thereafter be decoupled therefrom. Then the cylinders are again fully extended and connected to the frame at a lower position. Upon retraction of the cylinders in the second stage, the thruster can be further retracted into the shaft of the vessel, whereby the fame may come to extend above the main deck. Again, the cylinders occupy relatively little deck space on the vessel.